1. Field of the Invention
The present invention relates to control device and method for an automatic transmission.
2. Description of Related Art
In a recent automatic transmission of a vehicle, a hydraulic control circuit is switched to engage or disengage each corresponding one of a plurality of friction elements (e.g., hydraulic clutches and brakes) and thereby to select a gear stage from a plurality of gear stages. When the vehicle travels a downhill, a driver often tries to use an engine brake by removing his/her foot from an accelerator pedal. However, if a slope of the downhill is relatively steep, a sufficient engine brake cannot be attained by simply removing the foot from the accelerator pedal. In such a case, the driver turns off an over drive switch or changes a shift lever from a D range to an S range or L range to downshift the automatic transmission, so that the sufficient engine brake is exerted.
When the downshift is performed to increase the engine brake force based on a driver's demand for deceleration (based on the deceleration operation of the driver) at the undepressed state of the accelerator pedal, a gear ratio of the automatic transmission becomes larger due to the downshift. Therefore, it is required to increase a rotational speed of the engine in the amount that corresponds to the change amount of the gear ratio. However, since a throttle valve is normally closed in an operational mode, which requires such an engine brake force, the torque at an output side is transmitted to an engine side by torque transmission by friction elements for achieving a shift range after the downshift, resulting in an increase of an engine rotational speed. Therefore, an effect of the engine brake cannot be obtained at the required timing due to a longer gear shift period, or inertia torque occurs as braking torque of the vehicle due to the increase of the engine rotational speed, so that the engine brake force is temporarily increased, generating a shift shock. In addition, when the transmission torque of the friction elements is abruptly increased due to the controlling of the hydraulic pressure of the automatic transmission or the like, the engine rotational speed is quickly increased to shorten the shift time and in contrast, the braking torque is abruptly increased, further increasing the shift shock.
Japanese Patent JP-B2-2924463 teaches a gear shift control device of an automatic transmission. According to Japanese Patent JP-B2-2924463, an engine output increasing means of the gear shift control device temporarily increases an engine output when an automatic transmission is downshifted to a lower shift range in which an engine brake works substantially in the undepressed state of the accelerator pedal. A timer of the gear shift control device counts an elapsed time, which is counted up, i.e., is measured from a predetermined count start point such as a shift output point for switching, for example, a hydraulic control circuit at downshifting. An engine output increase control operation is started by the engine output increasing means based on the elapsed time period, which is counted by the timer. Specifically, the engine output increasing means increases the engine rotational speed after the time of starting the disengagement (slipping) of the off-going friction element at the higher speed gear stage but before the completion of the engagement of the on-coming friction element based on the elapsed time period, which is counted up, i.e., is measured with the timer. This timing for starting the engine output increase control operation is set based on the operational state of the vehicle (e.g., the hydraulic pressure in the hydraulic pressure control circuit and the engine rotational speed), which has the influence on at least one of the engagement/disengagement delay time period of the friction element and the engine output increase delay.
In addition, Japanese Patent JP-B2-2924463 teaches that the timing for starting the increasing of the engine rotational speed may be set in view of a delay in the disengagement/engagement of the corresponding friction element and/or a delay in the start of the increasing of the engine output to reduce the required gear shift time period while limiting the gear shift shock. Also, Japanese Patent JP-B2-2924463 teaches that it is desirable to control the opening degree of the throttle valve to increase the engine output in consistent with the timing of starting the slipping of the off-going friction element.
However, the delay in the engagement/disengagement of the friction element at the time of the downshift is changed not only depending on the hydraulic pressure of the hydraulic control circuit and the engine rotational speed but also depending on the vehicle speed and the torque applied to the friction element at the time of the downshift. Particularly, at the time of the downshift, the accelerator pedal is fully released to substantially, fully close the accelerator. Thus, it is required to consider the application of the engine side drive torque, which is equal to or lower than the road load torque (i.e., the torque that is required to travel at the constant speed at that time). Furthermore, the above delay is also influenced by the operational state in the slip control operation of the lock-up clutch. Thus, at the time of setting the timing for starting the engine output increase control operation, even when influences of the hydraulic fluid (oil) temperature and the engine rotational speed are considered, it is not always possible to start the throttle valve opening degree control operation (the engine output increase control operation) at the appropriate timing. Thus, the timing for starting the throttle valve opening degree control operation may possibly be deviated from the appropriate timing to cause the feeling of the acceleration and the gearshift shock to the driver due to the throttle valve opening degree control operation at the time of the downshift. Furthermore, in order to appropriately set the reference value of the timer in view of the influences of the hydraulic fluid temperature and the engine rotational speed, the reference value of the timer needs to be set based on the repeated experiments. Also, the reference value of the timer needs to be set when the way of depressurizing the hydraulic fluid needs to be changed due to a change in the specification of the transmission hydraulic pressure control device. Therefore, the logic is complicated, and the many parameters need to be set. Thus, the adjustment of the parameters takes a large amount of time.
In order to address the above disadvantage, JP-2006-A-69267 (corresponding to US 2006/0046896 A1) teaches another technique. According to this technique, a reduction in an input shaft rotational speed or a reduction in a gear ratio in a gear shift mechanism of the automatic transmission is sensed at the time of occurrence of the slipping of the off-going friction element at the higher speed gear stage at the time of the downshift to start the engine output increase control operation.
The inventors of the present application have found through the experimental study that even when the off-going friction element is released at the time of the downshift, the input shaft rotational speed or the gear ratio of the gear shift mechanism does not decrease in some cases (e.g., at the time of shifting from the third speed to the second speed in an automatic transmission that is similar to one shown in FIG. 3). In such a case, the engine output increase control operation cannot be started at the time of the downshift, and thereby the gear shift cannot proceed.
Furthermore, even in the case where the reduction in the input shaft rotational speed or the gear ratio is sensed at the time of the downshift, when the engine output increase control operation is started after the reduction of the input shaft rotational speed or the gear ratio to a corresponding predetermined value, a difference between the gear ratio at the time of starting the engine output increase control operation and the gear ratio after the gear shift (the input shaft rotational speed after the gear shift) is increased to cause lengthening of the entire gear shift time period.